1. Field of the Invention
The invention relates to polymethacrylate molded articles, with crosslinked polystyrene as a light-scattering agent to render them translucent (cloudy) rather than transparent.
2. Discussion of the Background
Glass substitutes based on methyl methacrylate (hereinafter, "methyl methacrylate glass materials") play an important role in the lighting industry and in the manufacture of light couples. For good room illumination it is necessary that the transparent or translucent fixture elements around the light sources, and dome elements, have fillers added to them which are not dissolved in the polymethyl methacrylate (PMMA) glass. The purpose of these fillers is to cloud the material and act as a scattering agent for through-transmitted light, while not greatly lessening the high light permeability of the material.
In addition to inorganic fillers, (e.g. barium sulfate), polystyrene has proved to be a particularly advantageous scattering agent, providing high scattering when employed as an additive. As described in DE 2,225,578 (U.S. Pat. No. 3,883,617) and DE 2,264,224 (U.S. Pat. No. 3,914,338), un-crosslinked polystyrene may be added to the monomer which is to be polymerized, which monomer is comprised essentially of methyl methacrylate. The polystyrene initially dissolves in the monomer, but separates out as the polymerization proceeds. It becomes finely and uniformly distributed, because polystyrene is insoluble in PMMA and the system forms two phases, comprising two respective incompatible polymers. Further processing of such polystyrene-clouded polymers, e.g. by injection molding or extruding, to mass produce molded or formed articles, has resulted in a quality degradation as to the light-scattering and light-permeability of the translucent material. This may be attributed to the formation of aggregates of the polystyrene particles, and to changes in the solubility and distribution of the polystyrene after it undergoes shear strain during processing in the PMMA matrix.
The disadvantages of this type can be alleviated according to the teaching of DE 2,146,607 and DE 2,264,224 if crosslinked polymers comprised essentially of styrene units are used. The crosslinked polystyrenes contain up to c. 5 wt. % of crosslinking agent.
According to DE 2,225,578, one can achieve the benefits of a crosslinked polymer clouding agent by using un-crosslinked polystyrene soluble in methyl methacrylate monomer, if the polystyrene also contains comonomer units which react to produce crosslinking at temperatures above 100.degree. C., in particular during further processing of the organic glass materials.
U.S. Pat. Nos. 4,876,311 and 5,004,785 describe translucent (clouded) plastic elements, e.g. such elements based on methacrylate resins, wherein the clouding agent comprises beads or fine particles of crosslinked material comprised essentially of monomer units containing aromatic groups, e.g. styrene or phenyl methacrylate, wherewith the material also comprises (in the polymer molecule) up to 20 wt. % of crosslinking agents.
Polymethacrylate clouded by means of un-crosslinked polystyrene, e.g. PMMA in the form of plate or sheet known as acrylic glass plate or sheet, has optical properties quite different from those of pure (and colorless) PMMA, such as light scattering (desired) and decreased transmissivity (not desired). The properties of such clouded PMMA are further diminished by tempering and by the action of light and weather; this degradation shows up in measurements of yellow value and of transmissivity. Addition of UV-protection agents and weathering protection agents, alone, to PMMA containing (un-crosslinked) polystyrene does not substantially alleviate this degradation.
It is sought to devise a polymethacrylate composition containing polystyrene as a clouding agent, which composition has high heat- and weather resistance, and which retains such resistance when subjected to, e.g., thermal forming operations to form light-scattering PMMA molded articles, and under conditions of use of such articles in illumination engineering, particularly outdoors.
Surprisingly, it was found that PMMA molded articles containing polystyrene light-scattering particles have high heat- and weather resistance, and retain such resistance, if the polymethacrylate composition contains:
a) crosslinked polymer particles comprised of units of phenyl group-containing monomers,
b) UV protection agents, and
c) radical scavengers.
Substances which may be used as crosslinked polymer particles comprised of units of phenyl-group-containing monomers include, in particular, crosslinked polystyrene, and crosslinked polymers which are comprised of phenyl-group-containing monomers.
The inventive polymethacrylate compositions and molded articles produced therefrom may also be colored, wherewith suitable colorants and dyestuff pigments may be added for this purpose.